PTH a. stimulates osteoblasts to lay down new bone. b....

$\mathrm{PTH}$
a. stimulates osteoblasts to lay down new bone.
b. reduces blood calcium levels.
c. stimulates calcitonin release.
d. is produced by the thyroid gland.
e. is released when blood calcium levels fall.

Key Concepts

Parathyroid Hormone (PTH)

PTH is a peptide hormone secreted by the parathyroid glands that plays a central role in calcium and phosphate metabolism. It acts by increasing blood calcium levels through mechanisms such as stimulating the release of calcium from bone, enhancing renal reabsorption of calcium, and promoting the activation of vitamin D in the kidneys, which in turn increases intestinal calcium absorption.

Calcium Homeostasis

Calcium homeostasis involves the regulation of serum calcium levels within a narrow range to support critical physiological processes such as muscle contraction, nerve conduction, and blood clotting. This balance is maintained by the coordinated actions of hormones like PTH and calcitonin, which have opposing effects on bone remodeling, kidney function, and calcium absorption in the gastrointestinal tract.

Bone Remodeling

Bone remodeling is the continuous process where old bone tissue is resorbed by osteoclasts and new bone tissue is formed by osteoblasts. Hormonal signals, particularly from PTH, regulate this process by influencing the activity of both cell types, ensuring that bone structure is maintained and that calcium is mobilized when needed for vital functions.

Calcitonin

Calcitonin is a hormone produced by the parafollicular cells of the thyroid gland. It functions to lower blood calcium levels by inhibiting osteoclast-mediated bone resorption, which is the process by which calcium is released from bones into the bloodstream, thereby providing a counterbalance to the actions of PTH in calcium regulation.

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